BEFORE. Over a few weeks, despite cleaning after each use, stains develop....
Run an inch or so of hot water in sink and pour generous amount of bleach into water... A cup or less should do it.
Lay boards into water & bleach and let soak an hour or so.
I added my sink liner which also needed brightening. Careful when removing... Do not splash. Drain sink and rinse off cutting boards.
AFTER! Clean & white!
Careful not to splash on your clothing or you can ruin a garment! Do not touch your clothes with your wet fingers. I do not think a guide exists that will fix the damage straight bleach can do!
Next time they look this way you will be able to restore easily!